Leaving USB drive plugged in to android causes battery drain even when sleep

If I accidentally leave a USB drive in my samsung galaxy S7+ it will drain the battery in a few hours even if it's not being used and the android goes "sleep". Why is this and and how to prevent it? E.g., I don't need to use the USB port when I'm not using the tablet 99.9% of the time. Also, the USB port seems to use a lot of battery in general even if I'm just playing a video off it or reading a pdf.